In CT imaging, cross sectional axial images or slices of a volume of interest (VOI), for example, of a patient's body or, more generally, of an object under inspection, are created by computer processing of X-ray attenuation data acquired at multiple view angles around an axis of rotation.
FIG. 1 is a schematic illustration of some basic features of a typical CT scanner 100 used for medical imaging. CT scanner 100 comprises a support rotor 102 mounted on a gantry (not shown) that carries an X-ray source 104 and a detector array 106, the latter being comprised of a plurality of rows 108 and columns 110 of closely spaced X-ray detector elements 112. Support rotor 102 is arranged for rotation in a direction indicated by an arrow 116 around a rotational axis coinciding with the Z-axis 134 of a coordinate system indicated by coordinate icon 114. It should be noted that coordinate system 114 rotates with rotor 102 so the Y axis remains pointing from the center of rotation to the X ray source 104 while the system is rotating.
A movable platform 118 is arranged to transport a patient 120 (or, more generally, an object being inspected) along the scanner Z-axis as indicated by arrow 122. A system controller 124 controls the operation of rotor 102, X-ray source 104, platform 118, as well as an image processor 126 connected to an output of detector array 106, and a display and storage unit 128.
One mode of operation is sometimes referred to as “step and shoot”. In this mode, platform 118 is held at a fixed axial position, and CT scanner 100 generates an X-ray beam 130 that emanates from a focal point 131 at source 104, and impinges on the detector elements 112 after passing through the patient's body. The X-ray beam may be fan-shaped, or as illustrated in FIG. 1, cone-shaped. Attenuation data from all the detector elements 112 is gathered for a succession of angular positions (or view angles), typically in the range of about 180 to about 360 degrees, as rotor 102 carries source 104 and detector array 106 around the subject.
The data collected from all the detector elements for all the viewing angles at a fixed axial position, generally referred to as projections, are computer-processed by image processor 126 to reconstruct one or more two-dimensional slice images. The slice images are displayed and stored by display and storage unit 128, which may include a computer monitor, or of any other desired and suitable display type, and a suitable data storage unit. In the case of a cone beam, a three-dimensional image may be created from the reconstructed axial slices. Combining the projection data from multiple axial positions obtained by moving platform 118 in steps allows creation of a larger three-dimensional image or scanned volume.
Alternatively, a CT scanner can be operated in a “spiral scan” mode in which the X-ray source and detector array rotate continuously, and the platform moves continuously along the axis of rotation.
A consideration in the use of CT for medical imaging is minimizing exposure to the radiation, both in and outside a VOI. Therefore, it is desired to position the patient optimally in the Z direction so the scanner covers the VOI. For good image quality, the scanner field of view (FOV) as defined below is desirably made large enough to encompass the portions of the subject radially extending outside of the precise VOI However, it is possible to scan the peripheral parts of the FOV with reduced dose (by using a butterfly or other filter) and/or reduced resolution. Further, CT scanners have a higher spatial resolution in the region close to the center of rotation. Consequently, for the VOI to have optimal image quality, it will generally be advantageous for the VOI to be located substantially at the center of the scanned volume. Further, with such a configuration it is possible to reconstruct the full images only within a limited volume around the VOI within the scan FOV whereas the rest of the data is used for image correction only.
In general, achieving a desired scan volume involves controlling the FOV of the scanner and selecting the number of axial positions at which projections are obtained. The FOV depends on the geometry of the scanner, and the collimation of the X-ray beam. For example, for an X-ray beam 130 that emanates symmetrically from X-ray source 104 relative to Z-axis 134, the size of the FOV is defined by a largest circle in a plane perpendicular to the z-axis (XY plane) that has its center on the rotation-axis and for which trajectories of X-rays from the X-ray source that are detectable by the azimuthal edges of the detector array are substantially tangent to the circle. This is indicated in FIG. 1 by dashed-line circle 132 having a radius R1 centered on the scanner Z-axis 134 for a symmetrically located detector array.
For a given cone beam geometry we define a volume field of view (VFOV) to be the volume that can be reconstructed from a circular scan, as depicted in FIG. 3B for a dual source scanner as explained hereinbelow.
For beams that are asymmetric relative the Y-axis, for a 360-degree rotation or larger scan, the larger angle relative to the radius determines the FOV. For scans of less than 360 degrees, the smaller angle determines the FOV.
Another known way to achieve a desired scan volume is to employ multiple spaced X-ray beams, either from multiple X-ray sources, or from a single source having multiple focal spots or X-ray beam emanation points. Numerous examples of X-ray emitters having multiple focal spots are known in the art, for example, U.S. Published Patent Application 2006/0285633, published Dec. 21, 2006 and entitled MULTIPLE SOURCE BEAN CT SCANNER (the '633 Patent Application), and U.S. Pat. No. 7,333,587 Issued Feb. 19, 2008, and entitled METHOD AND SYSTEM FOR IMAGING USING MULTIPLE OFFSET X-RAY EMISSION POINTS (the '587 Patent), the disclosures of which are incorporated by reference herein. The multiple X-ray beams may emanate from points spaced along the path of rotation, as in the case of emanation points 206a and 206b in FIG. 2 as discussed in detail below. Alternatively the emanation points may be spaced along a line parallel to the axis of rotation, as in the case of focal spots 306a and 306b in FIGS. 3A-3C. The spaced beams may be switched on and off at a high frequency as the rotor 102 rotates around the Z-axis to provide alternating partial projections that are computer-processed to provide a volumetric image.
According to conventional practice, a preliminary planar scan (with the rotor placed at fixed angle) is used to plan the positioning of the axial CT slices. In fan beam scanners, where the detector has a limited coverage in the Z direction, the patient is made to move in the Z direction during scan and the CT scanner is used substantially as a line scanner. In cone beam scanners having a sufficient number of detectors rows, the planar scan can be done by a single shot on a static patient or by a step and shoot procedure that comprises a small number of steps, depending on the area to be covered. For the single shot or step and shoot planar scan, the CT scanner is used as a digital radiography device. For planning CT scans of the body, “planar scan” radiographic images are typically acquired with the X ray source at 0 degrees or 180 degrees. For planning CT scans of the head, “planar scan” radiographic images are typically acquired with the X ray source at 90 degrees or 270 degrees.
To align the patient in left/right and up/down directions, two positioning scans would have to be performed (i.e., a first scan at 0 degrees for horizontal alignment and, and a second scan at 90 degrees for vertical alignment). This can be time consuming and an inconvenience for both the technician operating the scanner and the patient.
Typically, horizontal positioning (in/out along the Z-axis) is performed by computer control. Vertical alignment is done, if at all, by manual adjustment based on external laser markers projected on the patient. Motorized lateral (left/right) alignment capability is not provided in CT scanners available commercially; the patient supports do not even provide a degree of freedom for left/right positioning. Nevertheless left/right positioning is desirable for cardiac imaging since the heart is typically shifted to the left and a minor shift of the patient to the right is generally needed for proper alignment. This shift is sometimes achieved by sliding the patient sideways on the patient support.
It is also known that image resolution is highest and image artifacts are minimized near the center of the FOV. Being able to center the VOI in all three dimensions is advantageous for this reason as well.